Most electrical motors are driven by power electronic circuits such as power inverters. Modern electronic motor drives require accurate information of motor speed for control purposes. Traditionally, speed sensors are mounted on the motor shaft so as to provide speed information for control purposes. However, the use of speed sensors not only increases the cost of the drive system, it also reduces the reliability of the motor drive system. In addition, the extra requirements for a shaft extension and mounting arrangement of the speed sensor make it unattractive to use speed sensors. Reliability of speed sensors is an issue particularly in hostile working environments where wide temperature fluctuations, mechanical vibrations and dust are factors affecting the lifetime of the speed sensors.
In the last decade, much research effort has been devoted to control methods for electronic motor drives that do not require the use of speed sensors. Some methods involve the use of flux detection coils inside the motors, at the expense of costs and complexity. However, none of the existing speed sensorless methods can work at zero speed. For some so-called speed sensorless methods a common way to overcome the zero-speed limitation is to energize the motor so as to kick start the speed to become non-zero before the speed sensorless method becomes effective.